1. Field of the Invention
This invention relates to a variable focal length lens system in which the lower limit of the back focal length is not restricted, such as a compact zoom lens system used for 35 mm lens-shutter type camera.
2. Description of the Prior Art
Conventionally, there is known a two group type compact zoom lens system which comprises from the object side to the image side, a first lens group of a negative refractive power and a second lens group of a positive refractive power. On the other hand, in order to increase the zoom ratio, there are provided various types of zoom lens systems in which a lens group of a positive refractive power is located at the most objective side of the whole lens system, and in which a plurality of lens groups are shifted in the zooming operation. Almost all types of such zoom lens systems, however, are designed for a single lens reflex camera which requires sufficient room for the reflex mirror to swing therein. Thus, since the lower limit of the back focal length is restricted the total length of such zoom lens system, i.e., the distance from the object side surface of the lens element located at the most object side to the image plane, is relatively long.
Contrarily, as a compact zoom lens system in which the lower limit of the back focal length is not restricted, there are shown a type of lens system which includes, from the object side to the image side, a first lens group of a positive refractive power and a second lens group of a negative refractive power. This type is disclosed in Japanese Laid-Open Patent Application No. Sho 56-128911, and Sho 57-201213. In this type, however, since the shifting amount of the lens group shifted in the zooming operation is relatively long, the lens barrel of such a zoom lens system becomes big. Furthermore, in this type, since an aperture stop is shifted along the optical axis in the zooming operation, the construction of the lens barrel becomes complicated. Especially, if this type of zoom lens system is used for a 35 mm lens-shutter camera, such a camera becomes big and complicated because it requires to locate a plurality of shutter blades and its driving unit near the aperture stop.
Additionally, in a zoom lens system of this type, the effective F-number of the whole lens system inevitably changes in the zooming operation. If such a change is desired to be compensated, the real aperture size of the stop has to be changed in accordance with the zooming operation. Such a compensation is especially necessitated in the case that the exposure amount is controlled by a system other than the TTL light measurement.